Project Summary The development of trans-synaptic viral tracers is an important component of the BRAIN Initiative. At present, the lack of viral-based anterograde monosynaptic tracing tools with high signal strength and low toxicity is a gap in neuroscience. Herpes simplex virus (HSV) type 1 strain 129 (H129) is the most promising viral tool for anterograde neuronal tracing. However, current versions of genetically modified H129 viruses are limited by high virulence and toxicity, weak label signals that require immunostaining for detection, and time-dependent spread across multiple synapses. There is also a concern of the directional specificity of anterograde propagation of H129 recombinants, as they may propagate retrogradely. Investigators in the field have been working actively to develop improved versions of anterograde viral tracers, but progress has been limited. We have formed a strong interdisciplinary collaborative team composed of virologists and systems neuroscientists to develop anterograde monosynaptic recombinant H129 tracers with high signal strength and little or no toxicity for multi-species neural circuit analysis. Our published work and preliminary data establish the feasibility and key methodologies for the proposed research. We will capitalize on our established bacterial artificial chromosome (BAC) based system for rapid generation of recombinant H129 vectors and precise control of the H129 payload. We have a sound plan to reduce viral toxicity, enhance label signals and generate variants carrying different functional payloads. Our overall goal is to create a new set of safe, effective and validated anterograde-directed viral vectors that allow efficient labeling in monosynaptic projection targets of specific neuron types. These new tools will have a broad impact by enabling optical imaging, physiological recording, and activity manipulation of defined anterograde projection networks. For rapid resource sharing, we will create a service platform through the UCI Center for Virus Research to disseminate the new molecular tools to the neuroscience community.